1. Field of the Invention:
The present invention relates in general to an apparatus for monitoring the operation of making threaded tublar joints and in particular to an apparatus for controlling the applied torque and number of turns in such an operation.
2. Description of the Prior Art:
After a bore hole has been drilled to an oil or gas deposit, pipe strings are run into the bore hole for removing the oil or gas. The pipe strings are assembled at the well site from pipe sections each having external threads at one end and an internally threaded box member at the other end or external threads at both ends for use with an internally threaded coupling collar. As the pipe sections are connected together, they are run into the bore hole. Each pipe section is assembled to the top of the pipe string utilizing a power tongs unit which has a rotary jaw member for gripping the pipe and a motor for rotating the jaw member until the pipe section has been tightened to the desired degree. The joint must be tight enough to prevent leakage and to develop high joint strength but not so tight so as to damage the threads which can leak.
Early prior art techniques involved the determination of the applied torque to achieve the desired degree of tightness in the joints. For example, one technique involved the adjusting of the air supply maximum output pressure to a pneumatically driven tong motor to provide the required maximum torque as dictated by joint properties and tong power characteristics. Thus, the proper torque was developed when the tong motor stalled. Another technique involved the counting of the number of turns after the threads had been engaged at a "hand tight" point. These early techniques were unsatisfactory since the desired torque could be developed early as a result of damaged or dirty threads.
One prior art device which attempted to solve the problem included means for producing a signal indicating of the number of turns of the pipe section after measurement of a given torque by the torque measuring means. The device produced a warning of a bad joint upon the measurement of a predetermined torque before a measurement of a minimum number of turns had occured or the measurement of the maximum number of turns before the measurement of the predetermined torque had occurred. The device indicated a good joint upon the measurement of the predetermined torque value between the measurement of the minimum and maximum number of turns. Such a device is shown in U.S. Pat. No. 3,368,396 issued Feb. 13, 1968. Improvements to that device are disclosed in U.S. Pat. No. 3,606,664 issued Sept. 21, 1971, U.S. Pat. No. 3,745,820 issued July 17, 1973, and U.S. Pat. No. 4,091,451 issued May 23, 1978.